With its activities of drug trafficking, the Dojin-kai has allegedly been Japan's largest wholesale dealer in drugs since the late 20th century, after the disbanding of three other yakuza groups based in northern Kyushu; Tagawa-based Sadaoka-gumi, known as Japan's Methlord in the Showa era, which was crushed by the Taishu-kai, Okawa-based Hamada-kai, and Kumamoto-based Yamano-kai.

Notable ones include the Yama-Michi War (山道抗争, 1986–1987), where the organization substantially expelled the largest known Yamaguchi-gumi syndicate from the northern Kyushu area.

When the long-time boss Seijiro Matsuo suddenly announced his resignation in May 2006, a war broke out between the headquarters and a splinter group in Omuta who, naming themselves the "Kyushu Seido-kai," allegedly aligned themselves with the Yamaguchi-gumi, the Dojin-kai's rival and the largest yakuza syndicate in Japan.

The Fukuoka Prefectural Police discovered that the Seido-kai built a cenotaph in July 2009 in Omuta to pay tribute to their casualties.

[15] Kazuma Umeki, one of the top underbosses in the Seido-kai, was attacked with a 10-ton dump truck driven by a Dojin yakuza,[16] and in April 2011, two Seido seniors were shot with a revolver in Imari by a Dojin hitman,[17] and Sueharu Matsunaga, the head of the Seido's Matsunaga group, was killed with a bomb.

[18] Sometimes informally and mockingly dubbed the "Dojin Pharmacy" (Dōjin Yakkyoku), the Dojin-kai's activities, just same as its splinter group Seido-kai's, allegedly largely consist of drug trafficking.

In one notable case, as introduced in the "Drug Criminal Organization" section in the National Police Agency's 1991 Police White Paper, a Fukuoka-based 20-member small Dojin-affiliate had managed a systematic trafficking ring, selling methamphetamine to various drug-dealing yakuza organizations throughout the country.